1. Field of the Invention
The present invention generally relates to the improvement of the performance of semiconductor devices, such as multi-gate devices. More specifically, this invention relates to a method for mobility enhancement in these multi-gate devices.
2. Description of the Related Technology
The scaling down of silicon metal-oxide-semiconductor (MOS) devices has become a major challenge in the semiconductor industry. While previously, the shrinking of device features has already given many improvements in integrated circuit (IC) performance, nowadays new techniques, methods and materials are desirable beyond the 90 nm technology node.
One major problem in the scaling of conventional planar devices is the short channel effect, which starts to dominate over the device performance. A solution for this problem has been provided with the introduction of multi-gate field effect transistors (MUGFET). Due to their three dimensional architecture, with the gate wrapped around a thin silicon fin, an improved gate control (and thus less short channel effects) over the channel could be achieved by using multiple gates.
However, the introduction of this new device architecture has caused new problems. One of them is the mobility of the carriers in the device. Due to the different crystallographic orientations of the top surface and the sidewall surfaces of the fin, a difference in mobility is observed for electrons and holes. When using a standard (100) wafer surface with a <110> notch, the electron mobility in an nMOS MUGFET is compromised significantly due to the less favorable crystal orientation of the sidewall surfaces. The largest contribution to the overall drain current comes from these sidewall surfaces with (110)/<110> orientation/direction, which is the worst case for electron mobility. The (100)/<110> orientation/direction at the top surface of the nMOS MUGFET however is very beneficial for electron mobility, but this part only has a smaller contribution to the overall drain current. For a pMOS MUGFET however the opposite occurs. While the (110)/<110> orientation/direction at the sidewall surfaces is very beneficial for the hole mobility, the (100)/<110> orientation/direction at the top surface is less favorable.
Different possibilities have been suggested to enhance the mobility for both nMOS and pMOS MUGFET, all depending on the same principle, namely the introduction of strain in order to boost both electron and hole mobility. For these solutions, one should keep in mind that the semiconductor material of the fin/channel has its crystal orientation dependent sensitivity for charge mobility towards stress. For nMOS devices, on a standard (100)/<110> substrate, tensile stress in the parallel direction along the channel and compressive stress in the vertical direction perpendicular to the wafer surface is beneficial. For pMOS devices, the opposite occurs; compressive stress in the parallel direction along the channel and tensile stress in the vertical direction perpendicular to the wafer surface is beneficial.
Strain can be introduced in the fin/channel in two ways: a biaxial global strain or a uniaxial local strain.
The first is also referred to as substrate-induced strain. Biaxial global strain known in the prior-art, as explained in a paper by J. Wesler et al. “NMOS and PMOS Transistors Fabricated in Strained Silicon/Relaxed Silicon-Germanium Structures,” Electron Devices Meeting, 1992 Technical Digest (Dec. 13, 1992) pp. 31.7.1-31.7.3., is the introduction of a graded silicon germanium (SiGe) substrate with strained silicon (Si) surface layer. Due to the higher lattice constant of the relaxed SiGe compared to relaxed Si, the Si lattice is forced to align with the SiGe lattice and as a consequence, the Si surface layer will be under biaxial tensile strain and thus also the channel region formed in this strained Si layer. Due to the biaxial strain, this technique is advantageous for both pMOS and nMOS devices. One disadvantage, however, is the decrease in performance for shorter gate lengths.
Another possibility for introducing biaxial global strain is the use of a strained silicon on insulator (SSOI) substrate, as presented in a paper by E. Augendre et al. “On the scalability of source/drain current enhancement in thin film sSOI”, Proceedings of the 35th European Solid-State Device Research Conference 2005 (ESSDERC 2005, 12-16 Sep. 2005), pp. 301-304. By using a strained silicon-on-insulator (SSOI) substrate, the benefits of using a SOI substrate (improved isolation, reduction of parasitic capacitance) and the benefits of using strained silicon (mobility enhancement) can be combined. However, in this case only nMOS devices show better performance.
Another possibility is the use of silicon germanium on insulator (SGOI) substrate, as presented in a paper by T. Irisawa et al. “High current drive uniaxially-strained SGOI for pMOSFETs fabricated by lateral strain relaxation technique”, Symposium of VLSI Technology Digest of Technical Papers 2005 (14-16 Jun. 2006), pp. 178-179. A SGOI substrate combines the benefits of using an SOI substrate (improved isolation, reduction of parasitic capacitance) and using SiGe technology (mobility enhancement). However, in this case only pMOS devices show better performance.
For the introduction of uniaxial local strain, different approaches have been explored. One approach is the introduction of stress liners on top of the MUGFET device, as explained in a paper of Collaert et al., “Performance improvement of tall triple gate devices with strained SiN layers”, Electron Devices Letters IEEE (November 2005) Volume 26, Issue 11, pp. 820-82.
By depositing a contact etch-stop silicon nitride layer (CESL) on top of the transistors as a stress liner, strain can be introduced in the channel region. In the case of pMOS devices, both tensile and compressive layers show improved device performance, while for nMOS devices, only tensile layers give a higher performance. With the dual CESL approach, both types of stress can be introduced in the CMOS device. The main disadvantages of this technique are the additional process steps which are needed to deposit both compressive and tensile CESL.
A second approach for introducing uniaxial strain is the introduction of recessed, strained SiGe in the source and drain regions of a MUGFET device, as explained in a paper by P. Verheyen et al., “25% drive current improvement for p-type multiple gate FET (MuGFET) devices by the introduction of recessed Si0.8Ge0.2 in the source and drain regions.” Symposium of VLSI Technology Digest of Technical Papers 2005 (14-16 Jun. 2006), pp. 194-195. By etching the silicon substrate, recesses are formed and selective epitaxial SiGe is deposited in these recesses. Due to the larger lattice constant of SiGe compared to Si, the channel region in between the source/drain regions is put under uniaxial compressive stress, which is only favorable for pMOS devices.
The main disadvantage of all the proposed methods found in the prior art is that in most cases, only the mobility of one majority carrier type (e.g., electrons in the n-type MUGFET transistor in case of SSOI as stressor) is enhanced whereas the mobility of the other majority carrier type (e.g., holes in the p-type MUGFET transistor in case of SSOI as stressor) remains equal or is even degraded. Therefore, there remains a need for a method that enables mobility enhancement for both NMOS and PMOS simultaneously.
A second disadvantage of all the proposed methods found in the prior art is that solutions are sought to enhance the strain in the NMOS or in the PMOS transistor by introducing strain (putting more strain) by means of a stressor. Hence, there is a need for methods to selectively decrease the strain in a strained material in a controlled manner.